Open access is a movement that seeks to make scholarly materials such as published and unpublished articles, manuscripts, and datasets available to the public in a way that they never have been before.
Open access literature is usually defined as "digital, online, free of charge, and free of most copyright and licensing restrictions" and "is entirely compatible with peer review" (Suber, 2008).
It is made possible by the distribution power of the Internet and the consent of the author or copyright-holder (Suber, 2008).
Open access materials are reported to be downloaded more than traditionally published materials (Davis et al, 2008), to provide more opportunities for easier collaboration among researchers, and are even cited more than restricted (non-open access) materials in some disciplines (Lawrence, 2001).
This short animated video explains open access to research and why it is important.
Open Access 101, from SPARC from Karen Rustad on Vimeo.